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ORGANIZATION AND PRINCIPAL ACTIVITIES - Cost of revenue (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Subsidiary or Equity Method Investee [Line Items]      
Cost of revenues $ 32,160 $ 340,174 $ 413,971
Selling and marketing expenses 2,588 14,364 24,077
Research and development expenses 757 3,357 4,419
General and administrative expenses 6,832 36,145 23,012
Parent      
Subsidiary or Equity Method Investee [Line Items]      
Cost of revenues     23
Selling and marketing expenses     54
Research and development expenses     204
General and administrative expenses $ 0 109 5,394
Share-based compensation cost of the earnout shares   $ 109 $ 2,390